Abstract: In order to investigate the coarse mixing stage mechanism of the vapor explosion, an observable experiment equipment facility for low-temperature molten materials to be dropped into water was designed. In the experiment, molten material jet was injected into water to experimentally obtain the visualized information. The effects of metal thermal diffusivity, metal temperature, and coolant temperature were investigated. The experimental results show that the vapor explosion impulse is direct proportion to the arithmetic product of metal thermal diffusivity and temperature. The increase of the coolant temperature decreases the pressure of the vapor explosion. The metal thermal diffusivity has an important effect on the vapor explosion behavior and pressure.